The cardiovascular system has five main components: (1) a pump (i.e., heart); (2) a carrier fluid (i.e., blood); (3) a distribution system (i.e., arteries); (4) an exchange system (i.e., capillary network); and (5) a collection system (i.e., veins). Blood pressure is the driving force that propels blood through the distribution network. Stroke volume (SV) is the volume of blood pumped by the right or left ventricle of the heart in one contraction. Specifically, it is the volume of blood ejected from one of the ventricles during systole. The stroke volume is not all of the blood contained in the ventricle. Normally, only about two-thirds of the blood in the ventricle is ejected with each beat. The volume of blood that is actually pumped from the ventricle is the stroke volume (SV).
Cardiac output (CO) is determined by stroke volume (SV) and heart rate (HR) as follows:CO=HR×SV  (1)Typically, ventricular stroke volume (SV) is the difference between the ventricular end-diastolic volume (EDV) and the end-systolic volume (ESV).SV=EDV−ESV  (2)The EDV is the filled volume of the ventricle prior to contraction, and the ESV is the residual volume of blood remaining in the ventricle after ejection. In a typical heart, the EDV is about 120 ml of blood and the ESV about 50 ml of blood. The difference in these two volumes, 70 ml, represents the SV. Therefore, any factor that alters either the EDV or the ESV will change SV.
CO/SV measurement may provide a useful approach for characterizing cardiac pathology and predicting life-threatening events. Currently, there are three methods for CO and SV measurement and cardiac health status index calculation: (1) Fick method; (2) thermodilution method; and (3) angiographic image method. The Fick method is an invasive method that needs at least two blood samples and requires knowledge of oxygen consumption or VO2. Thermodilution is another invasive method that involves the injection of saline into the central venous pressure CVP/right atrial (RA) port. The saline flows through the right ventricle (RV) and cools the thermistor in the pulmonary artery (PA), at which the rate and degree can be utilized for CO and SV calculation. Angiographic method can also be utilized for CO and SV calculation, by estimating cardiac image volume at EoD (end of diastolic) and EoS (end of systolic) times. Such method usually requires extensive clinical experience and generates estimations with tremendous deviations.
These approaches have several shortcomings, such as requiring blood samples in the Fick cardiac output measurement, subjective and inaccurate cardiac output calculation based on images of EoD and EoS points in angiographic images, analysis deviation of measuring data and calculation in thermodilution-based cardiac output monitoring, etc. Further, current CO and SV calculation methods are based on complicated clinical devices and systems, such as invasive blood pressure catheters, X-ray image system, etc., which may increase patient risk due to tissue damage, radioactivity dose, complicated medical procedure, timing, sensitivity, false alarm, etc.
Most known CO/SV analyses are not accurate, since the corresponding data acquisition is not precise. For example, blood pressure measured in a noisy environment does not yield accurate data. Current image-based CO and SV calculations, such as those based on fluoroscopic and ultrasound images, calculate volume by measuring the size of the heart captured in the images in two or three dimensions. Such measurements are typically not accurate, especially for EoD and EoS stages, due to imprecise timing or gating. Current thermodilution methods typically use an injection waveform, which is invasive. So far, there are no known efficient quantitative and qualitative non-invasive clinical approaches for estimating CO and SV with good accuracy, sensitivity and stability.
Known less-invasive or non-invasive methods for estimating SV utilize blood stroke volume within local vessels to proportionally estimate the heart SV. The non-linear relationship between measured and actual heart SVs may result in huge calculation errors and subsequent false alarms, especially in critical care monitoring. There are no known accurate non-linear methods to bridge the local calculation index and true heart cardiac output.
Known methods for CO calculation do not fully or efficiently use vital sign signals (e.g., blood pressure), which are closely related to hemodynamic signals. Recently, some studies have been made on least invasive sensing of blood pressure to estimate CO. However, such methods also rely on catheter technologies, which limit the clinical applications. In addition, known clinical methods for CO/SV estimation require extensive clinical experience and knowledge to, for example, interpret parameters and evaluate calculation accuracy. This may impose some limitations for certain medical users. Some cardiac output determination methods rely on the quality of the sensor and acquired signals. Noisy environment or noisy data acquisition may cause false calculations and unreliable cardiac function estimation. Such noise may come from the power line, patient movement, or patient treatment process, such as pacing and drug delivery.